Slow Cooked Shredded Beef Tacos

1 1/2 lb beef chuck roast
1 medium onion sliced
1 C water
1 pkg taco seasoning mix
10oz taco sauce
1 can diced green chilies
taco shells
toppings: shredded cheese, lettuce, tomato, sour cream

Place beef and onion in slow cooker. Combine water and seasoning mix in small bowl. Pour over beef and onion. Cook on LOW 6-8 hours or until tender. Remove meat to cutting board; shred beef with 2 forks. Place beef in large bowl. Stir in taco sauce and chilies. Fill warmed taco shells with beef mixture. Serve with desired toppings.